A speed sensor is used as a speed measurement device of a moving object such as an automobile and a train in order to understand a motion circumstance of a target object. In a general moving object which moves by wheels or the like, a relative velocity is measured by measuring an angle of a rotation axis.
Frictional resistance by a surface state of a medium is applied at a contact portion of a road surface with a wheel and the like and thus a relative speed to the road surface without spinning may be measured.
However, an axial torque of a rotating object is greater than the frictional resistance in a low-p road which has a low frictional coefficient (mu (μ)) and the rotating object such as a wheel is in a state of easily sliding. Thus, it is impossible to accurately measure the relative speed to the road surface. The weather condition such as rain and snow causes a medium which causes the frictional coefficient to be decreased to be interposed between contact portions of the road surface and the rotating object, in addition to the low-μ road. Thus, a situation similar to that in the low-μ road occurs.
When a centrifugal force at a time when an automobile curve-travels causes the vehicle body to be inclined, and thus a wheel rolling direction and a vehicle traveling direction are different from each other, there are many traveling conditions which cause inaccuracy in speed measurement using the rotational axis to occur. In a linear motor car which travels in a state where the car body is raised, it is impossible to perform measurement by using a contact type speed sensor.
A non-contact type speed sensor is a sensor which measures a relative speed to a direct contact road surface by using the Doppler effect or a spatial filter type detection principle. A measuring device using light or laser is expensive due to a configuration of using an optical system and accuracy for mounting the measuring device is required.
A road surface state of large undulation due to dirt, gravel, a snow surface, and the like or correspondence of an optical device to resistance against stress due to vibration of a vehicle body is a difficult problem in a speed sensor having an optical system.
As a non-contact type speed sensor which does not use an optical system, a Doppler module using a radio wave has been developed. The microwave which is emitted from a patch antenna having an antenna half power width of 30 degrees and more has an incident angle range to the road surface, which is equal to or greater than 30 degrees and a few, and a traveling direction speed of a cos θ component of the microwave having an incident angle θ is distributed in accordance with the incident angle range. Since signal strength of a reflected wave is attenuated by speed distribution, sensitivity is degraded and measurement can be performed at only a close range.
As an improvement plan for the above problems, a dielectric lens is provided on a patch antenna, a range of directivity of a radiated radio wave is reduced, light is condensed at one spot or more, and thereby suppression of sensitivity degradation due to an incident angle distribution has been considered. PTL 1 and PTL 2 disclose the background in this technical field.
A technology in which a beam of a narrow angle is emitted to a patch antenna which is a primary radiator, or an open portion of a waveguide by using a dielectric lens having a large opening area and thus improvement of an antenna gain is obtained is disclosed (see Abstract of PTL 1).
In dielectric antennas disclosed in PTL 1 and PTL 2, an optical axis of the dielectric lens matches with a vertical axis of an opening surface of the primary radiator, and thus maximization of gain is obtained most conveniently.
However, it is necessary that a speed component cos θ in a traveling direction is obtained, in order to use the dielectric antennas in a speed sensor which is provided in a vehicle. Thus, a normal direction of the opening surface of the primary radiator is required to be installed so as to be inclined in accordance with an incident angle to the road surface. Accordingly, a frame having a slant surface of inclination θ is required in order to use a speed sensor which uses the dielectric antennas disclosed in PTL 1 and PTL 2. Since a frontal projected area in the traveling direction increases so as to be equal to or greater than sin θ of an opening diameter of the lens, a problem of road clearance with the road surface may occur, or an air resistance coefficient of a Cd value may increase and mounting adjustment of a speed sensor which is attached to the frame may be expected to be complicated.